Boreholes are drilled into the earth for many applications such as hydrocarbon production, geothermal production, and carbon dioxide sequestration. In order to efficiently use expensive resources drilling the boreholes, it is important for analysts to acquire detailed information related to the geologic formations being drilled.
Nuclear magnetic resonance (NMR) tools are one type of downhole tools that are particularly useful for performing detailed measurements of properties of hydrocarbon bearing formations. NMR measurements are used to determine among other things, porosity, hydrocarbon saturation, and permeability of rock formations. The NMR logging tools are used to excite the atomic nuclei of the fluids in the geological formations surrounding the borehole so that certain NMR parameters such as hydrogen density, longitudinal relaxation time (generally referred to in the art as T1) and transverse relaxation time (generally referred to in the art as T2) of the geological formations can be measured. From such measurements, the porosity, permeability and hydrocarbon saturation are determined, which provide valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons. The following references may be referred to for teachings with respect to performing NMR measurements: NMR LOGGING PRINCIPLES & APPLICATIONS by George R. Coates, Lizhi Xiao, and Manfred G. Prammer, Halliburton Energy Services Publication H02308 (1999); Nuclear Magnetic Resonance Petrophysical and Logging Applications by K.-J. DUNN, D. J. Bergman and G. A. Latorraca, PERGAMON (2002); and U.S. Pat. No. 6,051,973 to Manfred Prammer.
Unfortunately, tight rock and shale formations produce NMR signals with very short relaxation times T1 and T2. To resolve short T2 components that are less than 0.1 milliseconds in an NMR echo sequence, a very short interecho time (TE) of the order of a few micro-seconds is needed. Normally this is only feasible in laboratory core analyzers that use a relatively high NMR resonance frequency and consequently a relatively high static magnetic field (B0). “Micro-pores” associated with these kinds of rocks and with some other minerals typically contain water or hydrocarbon (very heavy oil) that, from an NMR perspective (hydrogen), appear almost like a solid component in a T2 distribution. Hydrogen in such “micro-pores” has a very fast decay rate generally due to the micro-pores providing a greater degree of surface area and thus a greater degree of pore wall interface than larger pores. Common NMR logging tools have difficulties and limitations to see this hydrogen signature in these micro-pores while they are readily seen, for example, in the producible water that is associated with larger pores. These difficulties are due to TE times that are relatively large due to the limitation of the technology used.